Joel Greenberg, research associate at the Field Museum in Chicago, has written an
utterly fascinating and thorough account of the forces that drove the passenger pigeon
to extinction. His goal is to use the centennial of this event as a teaching moment to
inform people about the passenger pigeon story and then to use that story as a portal
into consideration of current issues related to extinction, sustainability, and the relationship
between people and nature. It is hoped that this tragic extinction continues
to engage people and to act as a cautionary tale so that it is not repeated (p. xiii).

But what are the lessons to be learned from the passenger pigeon?Was its demise
a tragedy? Was it a tragedy of the commons? Are these events likely to be repeated?

The passenger pigeons extinction was almost completely unexpected because
these birds were not merely common but unbelievably numerousthe most abundant
bird species in the world. When European visitors to North America returned
with descriptions of the magnitude of passenger pigeon flocks, they were justifiably
met with incredulity, and they sometimes kept quiet, suspecting that no one would
believe them anyway. Yet credible accounts tell of virtual rivers of migrating birds.
One observer, English hunter and naturalist W. Ross King, recounted around 1860
that early in the morning he was awakened to see the sun obscured by millions of
pigeons . . . darting onwards in a straight line . . . in a vast mass a mile or more in
breadth, and stretching before and behind as far as the eye could reach. Swiftly and
steadily the column passed over with a rushing sound, and for hours continued in
undiminishedmyriads. . . . The duration of this flight being about fourteen hours . . . the
column (allowing a probable velocity of sixty miles an hour) could not have been less
than three hundred miles in length (qtd. on p. 5). Experts now estimate that this
flock would have contained about 3.7 billion (not a misprint) birds. About half a
century later, not a single pigeon was left.

Indeed, the passage of these pigeons could evoke apocalyptic descriptions.
When pigeons flew over Columbus, Ohio, in 1855, children screamed and ran for
home. . . . Horses bolted. A few people mumbled frightened words about the
approach of the millennium, and several dropped to their knees and prayed. . . . Day
was turned to dusk. After the flock passed over the ghostly town, the sunlight
illuminated a world plated with pigeon ejecta (p. 54). Pigeons, likened to a plague
of locusts, could spell doom. They could strip an area bare of crops, and the weight of
the roosting horde could destroy the value of a forest. They would eat almost any
crop, but especially liked fruit and mastacorns, beechnuts, and other tree nuts.
They competed directly with humansand their swine, which feasted on mastfor
these resources. Greenberg aptly titles one section They Made Great Havoc: Enemies
of Agriculture (p. 74).

Accordingly, most Americans saw these beautiful, even majestic birds as a dreadful
pestbut a tasty, nutritious pest that could feed a hungry nation. Pigeons were not a
high-priced delicacy; they were inexpensive food for the poor. Although Greenberg
notes that the masses of pigeons were viewed with wonder and hunger (p. 68),
he doesnt dwell enough on the latter point. It is important to realize how underfed
our ancestors were, something we can occasionally come close to measuring.
For example, John Komlos found that the average height of eighteen-year-old West
Point cadets in the middle of the 1800s was only sixty-seven inches (five foot seven),
and their average weight was a scrawny 126 poundsand these young men probably
came from more well-off families (The Height and Weight of West Point
Cadets: Dietary Change in Antebellum America, Journal of Economic History 47,
no. 4 [1987]: 897927).

So how did our underfed ancestors eradicate this species? Passenger pigeons
were unusually easy to catch, and accordingly people killed them in virtually every
way imaginable (p. 68). Many were shotalthough their market value was sometimes
so low that one hunter is quoted as saying that guns were preferred only when it
seemed certain that a single shot would bag five or more pigeons. Stool pigeons
were used as decoys to attract them, as were other forms of bait, but people more
commonly trekked into the pigeons nesting groundswhich could cover hundreds
of square milesand captured them in nets, clubbed them, or simply wrung the necks
of unlucky squabs who hadnt yet learned to fly or mature birds dislodged from trees
during nighttime raids. Such hunts became impressive affairs, with word of the
pigeons arrival spreading by telegraph and thousands of families converging. These
hunts often had the atmosphere and drunken gaiety (p. 94) of a picnic or a fair,
and on occasion special trains were dispatched to accommodate the influx. Catching
pigeons could be very remunerative, but it was brutal, messy work. After the hunt, the
birds were processed and eaten locally or shipped by rail to markets throughout the
country. (Oddly, Greenbergs delightful appendix, which covers a surprising array of
miscellany about the pigeonsincluding passenger pigeons in music, novels, poetry,
paintings and sculpture, movies, radio, TV, and theaterincludes not a single recipe.)

The eradication of pests is usually considered a public goodsomething that
benefits everyone, even those who dont pay for the servicewhich sometimes leads
governments to subsidize the activity. For example, until recent memory the eradication
of predators such as wolves was considered a good thing, with payments offered
for dead wolves. In 1850, Utah spent 15 percent of its territorial budget on wolf
bounties (Call of the Wild, The Economist, December 22, 2012). Pigeons were
widely viewed as pests, but this alone cannot explain their exterminationafter all,
eliminating other pests such as Yersinia pestis (the cause of plague), malaria-carrying
mosquitoes, and rats hasnt been as successful. Unlike most species, however, the
passenger pigeon apparently had an Achilles heel. Before pigeon hunters gained access
to modern transportation networks, the pigeons ecology of traveling and nesting in
gigantic multitudes was probably a strength rather than a weakness. Nut production
was very variable from year to year and location to location, but when mast was
available, pigeons could descend on the region, and their very numerousness meant
that local predators could make barely a dent in their population, including defenseless
young squabs, which parents left to their own devices before they could fly. This
strength was undone in the 1800s when human predation became more systematic,
and as flocks dwindled, they seem to have fallen below a threshold needed to
sustain this strategy. The decline itself likely fostered increased mortality, making
individual birds more vulnerable to predators (p. 195). Ironically, it appears that
the passenger pigeons economies of scale in foraging and nesting led to economies
of scale in hunting them. In addition, isolated refuges eventually disappeared as
once-inaccessible swamps were drained, with federal and state governments blessing
and encouragement.

The tragedy of the commons occurs when lack of property rights leads to the
unsustainable overuse of a resource, such as fish in the open sea or, perhaps, the
passenger pigeon. But it is not clear that the pigeons extinction was an instance of
this kind of tragedy. P. J. Hills recent analysis of the bison argues that the valuable
resource on the plains wasnt the buffalo, but rather the grass they ate. Hunting the
bison cleared the land for cattle, which were much more efficient in converting grass
into marketable meat (Are All Commons Tragedies? The Case of Bison in the
Nineteenth Century, The Independent Review 18, no. 4 [Spring 2014]: 485502).
Likewise, it is not clear that the passenger pigeon was the valuable resource farther
east; rather, it was the land on which the food they ate grew. Farmers were becoming
more and more efficient in turning this land into marketable calories, protein, and
nutrients, and the passenger pigeon was getting in the way. Attempts at domesticating
the passenger pigeon failed, but farmers constantly improved chicken breeds, successfully
turning grain into a reliable, low-cost source of eggs and meat.

For good or for ill, the displacement of the passenger pigeon reflects the
broader agricultural history of humankindtaking control of nature, driving out
wild species, and replacing them with domesticated animals that are more cooperative
at growing fat and being eaten. See, for example, Randall Munroes visualization,
based mainly on research by Vaclav Smil, of the global tonnage of
land mammals (Earths Land Mammals by Weight, xkcd, n.d., http://xkcd.com/
1338/). It shows that the entire mass of wild mammals such as elephants, bison, and
other wild animals is dwarfed by that of domestic mammals such as goats, pigs, and
sheep. Domesticated cattle collectively outweigh wild mammals by an immense
amount. But historically humans modified natural habitats even before they began
domesticating animals. For example, when New World species proved difficult to
domesticate, Native Americans adopted another strategyburning vast forests to
turn them into grasslands where large game could be hunted. Before the arrival of
Europeans, bison ranges covered most of the part of North America that would
become the United States east of the Mississippi. As Greenberg puts it, From the
perspective of most plants and animals, humansmodern and premodernmake
terrible neighbors (p. 202).

How likely is a repeat of something similar to the passenger pigeon episode?
Most modern extinctions have not been caused by an orgy of hunting; rather, they
have occurred to isolated populations, especially on islands, when other species invade
(e.g., pigs and crab-eating macaques raiding Dodo bird nests) or habitat is lost. I may
be wrong, but it seems unlikely to me that there will be other unwelcome but widely
eaten species that will be pursued as relentlessly as the passenger pigeonwe have
simply gotten much better at producing food cheaply without the need for the kind
of grubby predation practiced by the pigeons pursuers. Instead, as we have continued
to commandeer more of natures resources, we have gotten better at leaving room for
our favorite species to flourish (including some we used to dread, such as wolves)
simply because we sympathize with them and now have this luxury.

In a final twist, there are now plans for bringing back the passenger pigeon
reviving this extinct species using modern genomic breakthroughs. The logistics of
this rebirth have been discussed in TED Talks, at the Smithsonian, and in academic
conferences. (See the Long Now Foundations website, which outlines the great
passenger pigeon comeback, at http://longnow.org/revive/what-we-do/passengerpigeon/.)
But Greenberg, rather than ending on this optimistic note (which he
discusses in less than a page in an appendix), almost obsessively counts down the
deaths of the last few pigeons in his final few chapters, with an undercurrent implying
that every passenger pigeon hunter deserves some blame in their extinction.